Add and use a Transform data type (issue 344970043 by address@hidden)

[torsten . haemmerle]

https://codereview.appspot.com/344970043/diff/1/lily/stencil-integral.cc
File lily/stencil-integral.cc (right):

https://codereview.appspot.com/344970043/diff/1/lily/stencil-integral.cc#newcode1099
lily/stencil-integral.cc:1099: Offset center (robust_scm2double
(scm_cadr (rot), 0.0), robust_scm2double (scm_caddr (rot), 0.0));
+      center[X_AXIS] = s->extent (X_AXIS).linear_combination
(center[X_AXIS]);
+      center[Y_AXIS] = s->extent (Y_AXIS).linear_combination
(center[Y_AXIS]);
(or something similar)

Here, we've got the rotation centre problem as in issue 5346.

Standard regtests can't see it, but PNG based regtests show differences
for
skyline-grob-rotation.ly and
stencil-color-rotation.ly

skyline-boxes-ellipses.ly shows up, too, but there are only minimal
deviations due to rounding issues.

https://codereview.appspot.com/344970043/